The present invention relates to magnetrons and, more particularly, to a high power magnetron cooled by boiling at natural convection.
Magnetron generators and intensifiers are widely used to generate microwaves, for example in microwave ovens and in radar systems. Generally, only low power, air cooled magnetrons are used. These have powers up to about 3 kW and efficiencies as high as 60%-75%.
Higher power magnetrons must be cooled by the forced circulation of a cooling liquid through their anodes and cathodes. As a result, their efficiencies do not exceed 28%-30%, making them too inefficient for many otherwise desirable applications in chemical engineering, food processing, drying, disinfection of agricultural produce, etc. For example, the combined heating of wood by microwave radiation (heating the wood internally) plus hot gas or air (heating the wood externally) would reduce the drying time of green wood by many orders of magnitude and substantially decrease the total energy consumption.
The anodes of prior art magnetrons are of two types, rods and lamellae. Both types of anodes are used in low power, air cooled magnetrons. Only tubular rods are used as anodes in high power, liquid cooled magnetrons because of the difficulty associated with dissipating heat flux through lamellae. The tubular anodes used in liquid cooled magnetrons are hollow tubes of circular or rectangular cross section. These anodes experience intense heating during the operation of these magnetrons, because of bombardment by electrons emitted by the cathode, accelerated by the potential difference between the cathode and the anodes, and focused onto the zone of interaction by the magnetic field. Any nonuniformity in the flow of cooling liquid through the anodes and cathodes would lead to nonuniform heat transfer and damage to the anodes or cathodes. To make sure that the circulation of cooling liquid through the anodes and cathodes is uniform, powerful pumps and complex automatic control systems are necessary. Because the anode voltage is high (10 kV to 50 kV) and the supply voltage of the pumps is typically 220 V or 380 V, high-voltage plastic insulation is used to isolate the pumps electrically from the anodes and cathodes. These plastics generally are not heat resistant, and break down at temperatures above about 80.degree. C. The temperature of the coolant therefore must be kept below about 80.degree. C., with a consequent reduction in the thermodynamic efficiency of the cooling. Because ambient temperatures may be as high as about 50.degree. C., the heat exchangers of these cooling systems must be designed with large areas, to accommodate a temperature difference between the coolant and the surroundings of only 25.degree. C. to 30.degree. C.
There is thus a widely recognized need for, and it would be highly advantageous to have, a high-power liquid-cooled magnetron at least as efficient as known low-power air-cooled magnetrons. Preferably, the magnetron would require only built-in low-power pumps, or would rely exclusively on boiling at natural convection, thus not requiring pumps at all.